Dunno about the others, but Flikr has a paid "Pro" option which seems both really well done (not overpriced, gives very real benefits without undermining the free option, "safe" [doesn't delete all your data] if you can't pay for a while) and very popular. The free service is great, but there's real incentive to pay, and a huge percentage of the Flikr accounts I see are paid.

So Flikr do seem to have figured it out...